1, Field of the Invention
The present invention relates to memory devices including phase change memory materials with induced stress, and methods for manufacturing such devices.
2, Description of Related Art
In a phase change memory array including memory cells, each memory cell includes a phase change memory element. The phase change memory element is made of phase change materials that exhibit a large resistivity contrast between crystalline (low resistivity) and amorphous (high resistivity) states. Phase change materials may include alloys of materials such as germanium (Ge), antimony (Sb), tellurium (Te), gallium (Ga), indium (In), silver (Ag), selenium (Se), thallium (Ti), bismuth (Bi), tin (Sn), copper (Cu), palladium (Pd), lead (Pb), sulfur (S), and gold (Au). Phase change materials include GexSbxTex materials in general and Ge2Sb2Te5 materials in particular.
Ge2Sb2Te5 materials exhibit an amorphous phase and two crystalline phases over a range of temperatures. The first crystalline phase is the rock salt phase or the face centered cubic (fcc) phase. The second crystalline phase is the hexagonal phase. The fcc and hexagonal phases are formed when the temperature of the as-deposited amorphous material is increased to approximately 150° C. and 300° C., respectively (W. K. Njoroge et al., J. Vac. Sci. Technol. A, 20, 230 (2002)). The hexagonal phase is characterized by a higher density than the rock salt phase in the phase change materials, indicating that additional film shrinkage (2˜3%) would rise after high temperature (about 400° C.) back-end of line process (BEOL). The hexagonal phase transition would lead to grain-size variations and voids in the interface, resulting in a low yield and causing a reliability issue. Therefore the hexagonal phase formation is not desirable.
It is desirable to provide memory devices that inhibit the hexagonal formation in phase change materials, and methods for manufacturing such memory devices.